1. Field of the Invention
The present invention is related to alumina-based substrate materials for magnetic heads with low flying height and suitable for precision working such as plasma or thermally induced machining, intended for use in thin-film head sliders for hard disk drives in which the head is situated at a very small distance from the recording medium, transverse pressure contours (TPC) for thin-film tape recording devices, or advanced air bearings (AAB).
2. Description of the Prior Art
Alumina-based materials for magnetic heads have been designed considering, principally, the following characteristics:
(1) Chipping resistance in cutting, PA1 (2) Excellent machinability in cutting and lapping, and PA1 (3) Low friction in contact start-stop (CSS), which depends on the lubrication characteristics of the material.
The characteristic 1 above is indispensable to form an air bearing surface (ABS), i.e. the contact surface between the magnetic head and the recording medium. An ABS has been formed conventionally by machining work. A head with numerous chippings on the ABS therefore showed poor flying height and lubrication characteristics. Also, a material that tends to chip while being machined requires a low machining speed for precision machining for the ABS.
The characteristic 2 above relates chiefly to the throughput: a material difficult to cut and machine has requires a prolonged period for ABS machining.
The characteristic 3 above relates to the reliability of the head: a material with poor lubrication tends to show a low durability in repeated CSS.
Recent introduction of plasma machining, including ion beam etching (IBE) and reactive ion etching (RIE), or thermally induced machining including laser machining, instead of mechanical cutting and grinding, for ABS formation has rendered the characteristics 1 and 2 above rather unimportant.
The friction between the head and the medium, as referred to in 3 above, depends on the pressure and contact area of the two elements as well as the lubrication characteristics of the head material. For conventional large inductive heads, the contribution of the lubrication characteristics of the head material is important because of the relatively large contact area of the head with the medium, which translates into rather severe requirements upon the material. However, recent models of heads have contact areas with the medium smaller by a factor of 4 to 10 than conventional heads, as well as pressure against the medium lower by a factor of 3 to 10, thus making requirements on the lubrication characteristics of the material less stringent.
On the other hand, thin film heads have progressed from the conventional standard slider (100% slider, with a 4-mm thick substrate) to the micro-slider (70% slider, with a 2.8-mm thick substrate), nano-slider (50% slider, with a 2-mm thick substrate) and pico-slider (30% slider, with a 1.2-mm thick substrate). The femto-slider (10% slider, with a 0.4-mm thick substrate) is under development for future use.
Such miniaturization of the head has entailed reduction of the head flying height from 0.3 .mu.m to 0.2 and from 0.075 .mu.m to 0.05 .mu.m in the most recent models.
These changes represent improvements for higher recording density. The same objective has prompted the replacement of inductive heads by magnetoresistive (MR) heads. This has greatly changed requirements on head materials, of which some important ones are described below.
4 Magnetoresistive heads with low, stable flying height require a far more complicated head pattern than conventional heads. Such a head pattern can only be realized by plasma or thermally induced machining as mentioned earlier. Also, the pattern requires finishing precision of .+-.1-3 .mu.m and a surface roughness after machining less than about 1/10 times etching depth. The substrate should therefore have a low surface roughness after plasma or any other machining.
5 Thin film heads undergo magnetic annealing at 600.degree.-700.degree. C. during the production process, since sendust or other alloys that need magnetic annealing at high temperatures are used in the magnetic layer for MR heads.
This heat cycle reduces the bond strength between alumina and TiC particles and, consequently, makes the TiC particles tend to pull out during lapping of the ABS. This exerts an adverse effect on the lubrication characteristics in CSS as mentioned in 3 above. Therefore, a requirement for the substrate is that TiC particles do not pull out even after magnetic annealing.
6 It has recently been recognized that deformation of the substrate during magnetic annealing mentioned above causes deterioration of positioning accuracy in photolithography and of magnetic characteristics of the head. The substrate is therefore required to show no deformation during magnetic annealing.
7 Characteristics of MR heads are strongly affected by surface roughness because they can be as thin as several tens of nanometers. The device is formed on a substrate sputtered with an alumina film. Too thick an alumina film may lead to large warp during magnetic annealing at high temperatures. On the other hand, if the alumina film is thin, its quality will be easily affected by defects in the substrate. Since the size of defects in the substrate depends on the crystal grain size, the grains constituting the substrate should be as fine as possible.
The inventors have proposed in JP 86/50906 a sintered body consisting of 30-50 wt % of (TiC+TiO.sub.2) and a balance of alumina to which is added a small amount of a machinability agent such as MgO and Y.sub.2 O.sub.3, a sintering aid. The inventors have also disclosed in JP 90/62511 a magnetic head material consisting of alumina partially substituted with MgO or other machinability agents containing 5-45 weight % of titanium carbide, ZrO.sub.2, Y.sub.2 O.sub.3, and AlN. More recently, JP 94/2618 discloses a ceramic material for magnetic head sliders consisting of 100 weight parts of mixture of alumina with 5-40 wt % of titanium carbide, 0.01-5 weight parts each of Mn and Ti oxides, and Ga, Ba, Ce and Ni oxides.
While these materials do satisfy the requirements described in 1-3 above, their surface roughness after etching is as great as 1/4-1/3 times the etching depth, thus failing to meet the requirement of stable flying height as described in 4 above. When a substrate made of said materials is lapped to form the ABS after heat treatment at 600.degree.-700.degree. C., TiC particles pull-out to deteriorate the surface roughness by a factor of 2-5, which causes problems such as poor CSS characteristics or head crash as described in 5. As for the requirement 6 above, a substrate 3 inches in diameter and 2 mm thick heat-treated at 600.degree.-700.degree. C. shows a 5-20 .mu.m of warp which greatly impairs positioning accuracy in photolithography and magnetic properties. Said materials also have nonuniform grain size distributions which can bring about some large particles (20-100 .mu.m) in the substrate, which may introduce defects as in the case of large sizes. Therefore the materials do not meet the requirement 7 above. This is because of poor distribution of TiC in the material due to the absence of oxygen and nitrogen in TiC.
Thus, it is difficult to produce sliders for high-density recording such as those for TPC or AAB with high throughput using conventional Al.sub.2 O.sub.3 ---TiC substrates because of the inability to meet the requirements of items 3 to 7 in the Al.sub.2 O.sub.3 ---TiC substrates.